The chemiluminescence based enzyme linked immunosorbent assay (ELISA) has become a standard immunoassay technique in the laboratory. However, ELISA requires a relatively large volume of sample (˜50 μL) in order to detect an analyte at picomolar concentrations. Furthermore, ELISA, along with other immunoassay techniques, requires the use of non-specific binding methods and agents, such as horseradish peroxidase, in order to detect the analyte of interest.
Quantum dot-linked immunosorbent assay (QLISA) is an alternative technique where antibodies are conjugated to fluorescent nanoparticles (quantum dots) for detection and quantitation of the desired analyte. QLISA provides detection of an analyte at picomolar concentrations at sample volumes within the range of about 1 μL to about 5 μL, which is about a ten-fold decrease in sample volume as compared to ELISA. Moreover, in QLISA, the antibody for capturing an analyte of interest is covalently bound to the substrate, as opposed to non-specific binding methods used in traditional ELISA or other immunoassay techniques.
Techniques have been developed to perform the QLISA assay using analytes conjugated to solid supports suspended in a fluid medium, such as a liquid sample. However, prior to detection of the analyte from a liquid sample, the solid supports must be separated from the liquid in order to reduce the volume of the sample. A reduced sample volume ensures that all of the quantum dots can be excited using a focused beam of light and detected, thus ensuring the most accurate quantification method possible. Generally, the solid supports are filtered and then transferred into a holder for imaging and detection. Unfortunately, this transfer step may result in the loss of some of the spheres, thereby diminishing the accuracy of the results.